The present invention is directed to the art of chemical vapor deposition for forming thin films on a surface of a solid substrate. More particularly, the present invention pertains to a method for chemical vapor deposition using pulsed microwaves to dissociate and activate a vaporized solution of a precursor to form a thin film on a surface of a solid substrate at low temperature. The term low temperature means a temperature substantially reduced in comparison with substrate temperatures found in the prior art methods for chemical,vapor deposition. Thin films are formed on substrates, for example, in the manufacture of superconducting devices and integrated circuits.
Controlling substrate temperature to minimize the thermal budget while manufacturing integrated circuits is desirable, for example, to minimize reactions with reaction chamber surfaces, and to reduce the displacement of dopants to molecular lattices beyond a design range. The displacement of dopants alters the submicron dimensions of microcircuit architecture, affecting electrical characteristics of integrated circuits such as junction capacitance and switching speed. Several methods for controlling temperature in chemical vapor deposition processes for forming thin films on solid substrates are known.
U.S. Pat. No. 4,824,690 to Heinecke, et al teaches the use of pulsed radio frequency energy to form a plasma. The longer wavelengths of radio frequencies energize a volume much larger than the substrate, however, and only a fraction of the total energy is available to form the plasma which envelopes the substrate. While pulsed radio frequency energy may be used to reduce the power to the plasma, Heinecke's stated purpose is to match the dissociation rate of the plasma.
U.S. Pat. No. 4,937,094 to Doehler, et al describes the problem of heat buildup inside a chemical vapor deposition apparatus caused by generating plasma within a deposition chamber, and teaches the use of a cooling jacket to prevent contamination of a substrate from overheating. A disadvantage of this method is that the cooling jacket does not conduct heat away from the substrate itself. Furthermore, the gases surrounding the substrate act as an insulator, preventing efficient heat transfer to the cooling jacket from the substrate.
U.S. Pat. No. 5,015,493 to Gruen teaches the use of a pulsed plasma accelerated to a substrate by a DC voltage potential. A disadvantage of this method is the possibility of contamination of the substrate from materials dissociated from the DC electrode and related structures within the enclosure.
U.S. Pat. No. 5,024,182 to Kobayashi, et al teaches forming a plasma with microwaves, mixing the plasma with a reactive gas, and accelerating the mixture toward a substrate with a DC potential. A disadvantage of this method is the high substrate temperatures caused by the continuous microwave energy source. Another disadvantage of this method is the potential of contamination from the DC electrode and related structures within the enclosure.
U.S. Pat. No. 5,034,372 to Matsuno, et al describes the use of continuous microwaves to form and operate a plasma. This method also generates high substrate temperatures by use of the continuous microwave energy source, and may cause overheating of the substrate.
The disadvantages noted above of the currently known and practiced methods for forming thin films manifest the need for an improved method that provides sufficient energy to form a plasma while avoiding the high substrate temperatures that may adversely affect the performance of the devices being manufactured.